Heretofore, the best superconducting electronic devices have been fabricated as thin film epitaxial structures deposited on dielectric substrates or buffer layers, and the best substrate or barrier dielectric in thin film superconductor technology has been LaAlO.sub.3. Dielectric substrates and buffer layers for microwave devices and growth of high quality HSTC films must meet stringent requirements, including being a member of a cubic crystal system, exhibiting no twinning or strain and a close fit to the HTSC lattice parameters. They must also have a comparable temperature coefficient of expansion; a low isotropic dielectric constant; a low dielectric loss and no chemical reaction with HTSC and they must be mechanically strong. There have been problems with the use of LaAlO.sub.3. For one, its dielectric constant is too high, meaning that at high frequencies device features become unmanageably small. Another drawback of LaAlO.sub.3 is its anisotropic dielectric constant which creates very difficult device fabrication. It also undergoes a phase transition leading to twinning and stress.
Many of these problems were overcome by the above-referenced CECOM Docket No. 5433, but the rare earth metal compounds of that invention are magnetic, particularly at low temperatures, and losses may arise from the use of rare earth metals in those compounds. Magnetic losses were overcome by the above-referenced U.S. Pat. No. 5,814,584 which also disclosed lattice parameters in the A.sub.2 MeSbO.sub.6 compounds that provided a comparable or better fit to YBa.sub.2 Cu.sub.3 O.sub.7-.delta. (YBCO) than the compounds disclosed in CECOM Docket No. 5433.
One unsolved problem remains ionic diffusion, which can occur across the boundary between the substrate/barrier and the YBCO, scandium (Sc), indium (In) and gallium (Ga) compounds disclosed in U.S. Pat. No. 5,814,584, issued on Apr. 29, 1998, which may diffuse across and substitute for copper (Cu.sup.2+). This diffusion will reduce both the critical temperature, or T.sub.c, and the critical current density, or J.sub.c, while increasing the surface resistance, or R.sub.s, all of which are detrimental to microwave device operation. In order to overcome the drawbacks and limitations of past devices utilizing LaAlO.sub.3 or MgO substrates and buffered YSZ or sapphire, as well as the tendency for ions to diffuse across the boundary between the substrate/barrier and the YBCO, antimonates with ordered perovskite structures have been investigated because these materials provide a relatively low dielectric loss of approximately 1.times.10.sup.-3 and provide isotropic dielectric constants. The term "low dielectric loss," as used throughout this disclosure, is defined as any dielectric loss lower than 1.times.10.sup.-2. The term "low dielectric constant," as used throughout this disclosure, is defined as any dielectric constant lower than 26, and, in this invention ranges from 9-16.2, within an experimental error of +/-5%.
The present invention solves the barrier diffusion problem by deposition of thin film compounds by pulsed laser deposition in the system A.sub.4 MeSb.sub.3 O.sub.12 where A is either barium (Ba) or strontium (Sr) and Me is an alkali metal ion selected from the group consisting of lithium (Li), sodium (Na) and potassium (K) in which these ions may diffuse across the substrate/film interface without adversely impacting the T.sub.c and J.sub.c characteristics, without the drawbacks and limitations of previous compounds. The ions of the compounds of the present invention may also diffuse across the interface between the substrate/barrier and YBCO, except that these ions, depending upon which YBCO ions they substitute for, may increase T.sub.c and J.sub.c for small concentrations, and suffer from none of the disadvantages and drawbacks of the compounds utilizing LaAlO.sub.3 or MgO substrates and buffered YSZ or sapphire.
Crystal structures of the compounds in the system Ba.sub.4 MeSb.sub.3 O.sub.12, where Me is Li and Na have been previously determined by x-ray and neutron diffraction and both of these compounds were found to be cubic, to belong to space group Im3m, to be perovskites and to exhibit an Me:Sb ordering of 1:3 on B sites. Also, Sr.sub.4 NaSb.sub.3 O.sub.12 has been reported in the literature as being monoclinic space group P2.sub.1 /n as determined from x-ray and neutron diffraction studies. The inventors herein have made different findings. Further, the prior art does not disclose uses of these compounds in HTSC and hybrid microwave devices made herein.
The following references and publications, describe the prior art in this area:
A. Tauber, et. al., abstract entitled "Sr.sub.2 ReSbO.sub.6 ; Re=Rare Earth, Barrier/Dielectric Layers and Substrates for Thin Film High T.sub.c Superconductors for Microwave Applications," published in Abstracts of Materials Research Society 1994 Fall Meeting, p.292, Nov. 27-Dec. 2, 1994, Boston, Mass.;
S. C. Tidrow, et. al., paper entitled "HTSC Substrate and Buffer Compounds, A.sub.2 MeSbO.sub.6 Where A=Ba, Sr and Me=Sc, In and Ga," published in Physica C, presented at 1995 Material Research Society's Spring Meeting held in San Francisco, Calif.;
A. J. Jacobsen, et. al., 30 Acta Crystalligrahica, 1705-1711 (1974);
J. A. Alonso, et. al., 22 Materials Research Bulletin, 69-74 (1987);
K. P. Reis, et. al., preprinted in Texas Center for Superconductivity at University of Houston and printed at 49 Acta Crystalligrahica, 1585-1588 (1993);
J. A. Alonso, et. al., 84 Journal of Solid State Chemistry, 16-22 (1990);
P. Woodward, et. al., 9 Journal of Material Research, 2118-2126 (1994);
R. D. Shannon et al "Dielectric constants of yttrium and rare-earth garnets, the polarizability of gallium oxide and the oxide additivity rule," (1990); and
R. D. Shannon "Dielectric polarizabilities of ions in oxides and fluorides," (1993).